Neuroendocrine cultured cells counteract persistent prion infection by down-regulation of PrPc.

Cell models for prion diseases are mainly of neuronal origin. However, the pathological isoform PrP(Sc) of cellular prion protein (PrP(c)) and prion infectivity are found in a variety of extraneural tissues in prion diseases. Although many cell types are not able to propagate PrP(Sc), little is known about cellular mechanism counteracting prion infection. It is desirable to identify neuronal or non-neuronal cell models that restrict PrP(Sc) generation or propagate PrP(Sc) only transiently. Neuroendocrine cells are derived from tumours forming the interface between endocrine and nervous system. We investigated the susceptibility of such murine cell lines to prion infection, which were in principle able to transiently propagate PrP(Sc). Surprisingly and in contrast to neuronal cells prion infection was abrogated by rapid and PrP(Sc)-specific down-regulation of PrP(c) expression upon exposure to prion-infected material. Cell lines described here provide novel models for studying PrP(c) regulation and intrinsic cellular defence mechanisms upon prion exposure.

Strategies for eliminating PrP(c) as substrate for prion conversion and for enhancing PrP(Sc) degradation.

Prion diseases are fatal neurodegenerative infectious disorders for which no therapeutic or prophylactic regimens exist. Our work aims to eliminate PrP(c) as substrate for the conversion into PrP(Sc) and to increase the cellular clearance capacity of PrP(Sc). In order to achieve the first objective, we used chemical compounds which interfere with the subcellular trafficking of PrP(c), e.g. by intracellular re-routing. Recently, we found that PrP(c) requires cholesterol for cell surface localisation. Treatment with mevinolin significantly reduced the amount of cell surface PrP(c) and led to its accumulation in the Golgi compartment. These data show that cholesterol is essential for the cell surface localisation of PrP(c), which is in turn known to be necessary for the formation of PrP(Sc). Another anti-prion strategy uses RNA and peptide aptamers directed against PrP(c). We have selected peptide aptamers using a constrained peptide library presented on the active site loop of the Escherichia coli protein TrxA in a Y2H screen. Several peptides reproducibly binding to PrP(c) in several assays were identified. Preliminary data indicate that selected peptide aptamers are able to interfere with prion propagation in prion-infected cells. To obtain additive effects we have tried to clarify cellular mechanisms that enable cells to clear prion infectivity. This goal was achieved by selective interference in intracellular signalling pathways which apparently also increase the cellular autophagy machinery. Finally, we have tried to establish an active auto-vaccination approach directed against PrP, which gave some positive preliminary results in the mouse system. This might open the door to classical immunological interference techniques.

Autophagy induction by trehalose counteracts cellular prion infection.

Prion diseases are fatal neurodegenerative and infectious disorders for which no therapeutic or prophylactic regimens exist. In search of cellular mechanisms that play a role in prion diseases and have the potential to interfere with accumulation of intracellular pathological prion protein (PrP(Sc)), we investigated the autophagic pathway and one of its recently published inducers, trehalose. Trehalose, an alpha-linked disaccharide, has been shown to accelerate clearance of mutant huntingtin and alpha-synuclein by activating autophagy, mainly in an mTOR-independent manner. Here, we demonstrate that trehalose can significantly reduce PrP(Sc) in a dose- and time-dependent manner while at the same time it induces autophagy in persistently prion-infected neuronal cells. Inhibition of autophagy, either pharmacologically by known autophagy inhibitors like 3-methyladenine, or genetically by siRNA targeting Atg5, counteracted the anti-prion effect of trehalose. Hence, we provide direct experimental evidence that induction of autophagy mediates enhanced cellular degradation of prions. Similar results were obtained with rapamycin, a known inducer of autophagy, and imatinib, which has been shown to activate autophagosome formation. While induction of autophagy resulted in reduction of PrP(Sc), inhibition of autophagy increased the amounts of cellular PrP(Sc), suggesting that autophagy is involved in the physiological degradation process of cellular PrP(Sc). Preliminary in vivo studies with trehalose in intraperitoneally prion-infected mice did not result in prolongation of incubation times, but demonstrated delayed appearance of PrP(Sc) in the spleen. Overall, our study provides the first experimental evidence for the impact of autophagy in yet another type of neurodegenerative disease, namely prion disease.

The tyrosine kinase inhibitor imatinib mesylate delays prion neuroinvasion by inhibiting prion propagation in the periphery.

Prion diseases are fatal neurodegenerative disorders with no effective therapy. A hallmark of prion disease is the conversion of the normal cellular form of prion protein PrP(C) into a disease-associated isoform PrP(Sc). The authors recently have shown that a tyrosine kinase inhibitor, imatinib mesylate, induces clearance of PrP(Sc) via specific inhibition of c-Abl in prion-infected cell culture models. In this study, the authors assessed the in vivo effects of imatinib mesylate on prion disease using a scrapie-infected mouse model and further investigated prion infectivity of the drug-treated scrapie-infected neuroblastoma (ScN2a) cells. The authors found that imatinib mesylate abolished prion infectivity to almost undetectable level in ScN2a cells and the level of PrP(Sc) was significantly decreased by the drug in scrapie-infected mouse spleens as well as in ScN2a cells. Moreover, the drug treatment at an early phase of peripheral scrapie infection delayed the appearance of PrP(Sc) in the central nervous system (CNS) and onset of clinical disease in mice. However, neither intraperitoneal nor intracerebroventricular delivery of the drug exerted any PrP(Sc) clearance effect in the CNS.

Recognition of lumenal prion protein aggregates by post-ER quality control mechanisms is mediated by the preoctarepeat region of PrP.

Prion diseases are fatal transmissible neurodegenerative disorders linked to an aberrant conformation of the cellular prion protein (PrP(c)). We have shown previously that the chemical compound suramin induced aggregation of fully matured PrP(c) in post-ER compartments, thereby, activating a post-ER quality control mechanism and preventing cell surface localization of PrP by intracellular re-routing of aggregated PrP from the Golgi/TGN directly to lysosomes. Of note, drug-induced PrP aggregates were not toxic and could easily be degraded by neuronal cells. Here, we focused on determining the PrP domains mediating these effects. Using PrP deletion mutants we show that intracellular re-routing but not aggregation depends on the N-terminal PrP (aa 23-90) and, more precisely, on the preoctarepeat domain (aa 23-50). Fusion of the PrP N-terminus to the GPI-anchored protein Thy-1 did not cause aggregation or re-routing of the chimeric protein, indicating that the N-terminus is only active in re-routing when prion protein aggregation occurs. Insertion of a region with a comparable primary structure contained in the PrP paralogue prnd/doppel (aa 27-50) into N-terminally deleted PrP re-established the re-routing phenotype. Our data reveal an important role for the conserved preoctarepeat region of PrP, namely controlling the intracellular trafficking of misfolded PrP.

The tyrosine kinase inhibitor STI571 induces cellular clearance of PrPSc in prion-infected cells.

The conversion of the cellular prion protein (PrP(c)) into pathologic PrP(Sc) and the accumulation of aggregated PrP(Sc) are hallmarks of prion diseases. A variety of experimental approaches to interfere with prion conversion have been reported. Our interest was whether interference with intracellular signaling events has an impact on this conversion process. We screened approximately 50 prototype inhibitors of specific signaling pathways in prion-infected cells for their capacity to affect prion conversion. The tyrosine kinase inhibitor STI571 was highly effective against PrP(Sc) propagation, with an IC(50) of < or =1 microM. STI571 cleared prion-infected cells in a time- and dose-dependent manner from PrP(Sc) without influencing biogenesis, localization, or biochemical features of PrP(c). Interestingly, this compound did not interfere with the de novo formation of PrP(Sc) but activated the lysosomal degradation of pre-existing PrP(Sc), lowering the half-life of PrP(Sc) from > or =24 h to <9 h. Our data indicate that among the kinases known to be inhibited by STI571, c-Abl is likely responsible for the observed anti-prion effect. Taken together, we demonstrate that treatment with STI571 strongly activates the lysosomal degradation of PrP(Sc) and that substances specifically interfering with cellular signaling pathways might represent a novel class of anti-prion compounds.